Interpretive Summary: The bean leaf beetle is an important soybean insect pest in the U.S. Insecticides are the primary means for reducing the impact of bean leaf beetles on soybean yield and quality. Cultural practices that affect soybean nodulation may have the added function of reducing bean leaf beetle populations in soybeans which in turn would allow farmers to manage populations of these insects without using insecticides. Because first generation adults begin to emerge from soybean fields in mid-July and because soybean root nodules are thought to be an important food source for bean leaf beetle larvae, it is possible that soil management tactics that affect soybean nodulation such as starter N fertilizer application could affect bean leaf beetle populations which in turn may impact yield production. Our research demonstrated that soybeans under the high and medium N input treatments fixed atmospheric N at rates equal to those seen in the no N input treatments, that first generation larvae sampled from the medium and no N input plots had larger body size than larvae sampled from the high N input plots, and that head capsule widths of first generation beetles that emerged from medium N input treatments were significantly larger than beetles that emerged from the other N input treatments. We conclude that the relationships between soil N management, soybean N relations, and bean leaf beetle biology are complex, and that the elucidation of these relationships will require additional research.

Technical Abstract:
Bean leaf beetles (Cerotoma trifurcata) are serious insect pests of soybeans (Glycine max). This study was conducted to determine if soil nitrogen (N) input treatments would impact the biology of this economically-important pest species. The experiment was conducted in the soybean phase of a long-term corn/soybean rotation study near Brookings, SD. Soil N input treatments were: corn fertilized for a yield goal of 8.5 Mg ha-1 (high N input), 5.3 Mg ha-1 (medium N input), or corn not fertilized (no N input). Corn and soybean plots also were treated with banded starter fertilizer (112 kg ha-1) as 14-16-11, 7-16-11, or 0-16-11 elemental N-P-K on the high N, medium N, and no N input treatments, respectively. Soybeans grown on high N input plots showed higher shoot NO3-N concentration than plants grown on the no N input treatments from the beginning bloom to the beginning pod development stages. Shoot ureide-N concentrations were not significantly different across N input treatments. First generation larvae from the medium and no N input plots had significantly larger body size than larvae from the high N input plots. Head capsule widths of first generation beetles from medium N input plots were significantly larger than beetles that emerged from the other N input plots. Our findings suggest that the soil N management treatments imposed during the 2005 growing season had little effect upon ureide-N concentration, which in turn suggests that there were few differences in soybean root nodulation across soil N management treatments. Because larvae are thought to feed upon root nodules, it was surprising to find significant soil N input treatment effects on bean leaf beetle larval characteristics. We conclude that the relationships between soil N management, soybean N relations, and bean leaf beetle biology are complex, and that the elucidation of these relationships will require additional research.